In a mobile radio communication system, like a cellular phone communication system, which communicates between a base station fixed on the ground and a mobile station having a relative speed to the base station, the mobile station makes a frequency of the reference clock which is equipped in the mobile station follow a frequency of the base station reference clock which is superposed on a signal (a downlink signal) from the base station to the mobile station (frequency capture and frequency following). And, the mobile station performs signal processing of the downlink signal based on the reference clock following the base station reference clock. Also, the mobile station creates a signal (an uplink signal) to the base station using the same reference clock and transmits that to the base station.
As an example of a mobile station used in this kind of mobile radio communication system, a cellular phone, which adopts a CDMA (Code Division Multiple Access) system and a WCDMA (Wideband Code Division Multiple Access) system described in Japanese Patent Application Publication No. 2001-157263, Japanese Patent Application Publication No. 2000-269881, Japanese Patent Application Publication No. 2004-104223, or Japanese Patent Application Publication No. 2004-333403, is known (hereinafter, it is also simply referred to a CDMA system cellular phone and a WCDMA system cellular phone).
First, as the first related art, the CDMA system cellular phone described in Japanese Patent Application Publication No. 2001-157263 or the like will be described with reference to FIG. 10.
The CDMA system cellular phone includes an antenna 1, a low noise amplifier 2, a down-converter 3 of heterodyne system and an RXAGC (Receiver Automatic Gain Control) amplifier 4 as shown in FIG. 10. The CDMA system cellular phone also includes a quadrature demodulator (DEM) 5, an A/D (analog/digital) converter 6, a PLL (Phase Locked Loop) 7 and a reference clock generator (TCXO (Temperature Compensated Crystal Oscillator)) 8. Also, the CDMA system cellular phone includes a delay profile searcher 9, a finger circuit 10, a timing generator 11, a frequency offset estimator 12, a RAKE circuit 13, a speech processor 14, a codec circuit 15, a speaker 16 and an accumulator 17. The CDMA system cellular phone also includes a microphone 18, a channel codec 19, a D/A (digital/analog) converter 20, a quadrature modulator (MOD) 21, a TXAGC (Transmitter Automatic Gain Control) amplifier 22, an up-converter 23 and a power amplifier 24.
Next, each part will be described. The PLL 7 generates local frequency signals for the down-converter 3, the up-converter 23, the quadrature demodulator 5 and the quadrature modulator 21 by using output of the reference clock generator 8 as a reference clock. A receiving signal received at the antenna 1, as a downlink signal from a base station, is converted into an intermediate frequency signal in the down-converter 3 after amplified in the low noise amplifier 2. After level control is performed in the RXAGC amplifier 4, the converted intermediate frequency signal is converted into a reception analog baseband signal by quasi-synchronous demodulation in the quadrature demodulator 5. The reception analog baseband signal is further converted into a reception digital baseband signal in the A/D converter 6, and the converted reception digital baseband signal is input to the delay profile searcher 9 and the finger circuit 10. Here, a base station reference clock from a base station is superposed on the reception digital baseband signal.
The delay profile searcher 9 generates a frame timing correction time for the reception digital baseband signal input sequentially, based on a frame timing signal given from the timing generator 11, and outputs the frame timing correction time to the timing generator 11. The timing generator 11 generates an ideal frame timing signal by using the reference clock output by the reference clock generator 8. And, the timing generator 11 generates the frame timing signal by performing correction with the addition of the frame timing correction time to the ideal frame timing signal, and supplies the frame timing signal to the delay profile searcher 9 and the finger circuit 10. When the frame timing signal is input newly, the delay profile searcher 9 calculates a difference in timing and supplies this to the timing generator 11 as a new frame timing correction time. By repeating such correction process, an accurate frame timing signal is always supplied to the delay profile searcher 9 and the finger circuit 10.
The finger circuit 10 includes a plurality of fingers for separating receiving signals being dispersed and delayed in multipath components. The finger circuit 10 demodulates the reception digital baseband signal in each finger, based on the input frame timing signal, and sends the outputs of the fingers to the RAKE circuit 13. The finger circuit 10 also extracts pilot data included in the reception digital baseband signal with respect to each finger, and outputs to the frequency offset estimator 12. The frequency offset estimator 12 calculates a frequency offset value for each finger, based on the pilot data with respect to each finger input from the finger circuit 10, and to outputs the frequency offset value for each finger to the RAKE circuit 13. By performing weighted synthesis of the output signals with respect to each finger from the finger circuit 10, which is based on the frequency offset value with respect to each finger input from the frequency offset estimator 12, the RAKE circuit 13 generates reception demodulated data in which the fading effect has been reduced. The generated reception demodulated data is input to the speech processor 14, and further, that is decoded by the speech processor 14. The decoded output is input to the codec circuit 15. The codec circuit 15 converts the output of the speech processor 14 into an analog signal, and outputs that from the speaker 16 as sound.
The frequency offset estimator 12 also performs weighted synthesis of the frequency offset value for each finger of the finger circuit 10 and outputs that to the accumulator 17 as a synthetic frequency offset value. The accumulator 17 adds the input synthetic frequency offset value and the present output value, and outputs the addition result to a frequency control terminal of the reference clock generator 8. The reference clock generator 8 changes an oscillation frequency according to the output value of the accumulator 17 which is input to the frequency control terminal. In this way, the output frequency of the reference clock generator 8 follows the reference clock frequency from a base station which is superposed on a receiving signal and changes (frequency capture and frequency following).
On the other hand, a voice signal input from the microphone 18 is converted by the codec circuit 15 into a digital signal, and the converted signal is encoded in the speech processor 14, and the encoded signal is input to the channel codec 19 as transmitting data. The transmitting data is converted by the D/A converter 20 into a transmission analog baseband signal after encoded by the channel codec 19, and is further modulated to an intermediate frequency signal by the quadrature modulator 21. The intermediate frequency signal is converted by the up-converter 23 into a transmitting frequency after being amplified in the TXAGC (Transmitter Automatic Gain Control) amplifier 22. The converted signal with transmitting frequency is transmitted from the antenna 1 as a transmitting signal after further amplified by the power amplifier 24.
As mentioned above, a local frequency signal supplied to the quadrature modulator 21 and a local frequency signal supplied to the up-converter 23 are generated by the PLL 7 which uses output of the reference clock generator 8 as a reference clock. Accordingly, a frequency of the transmitting signal will also change according to the output frequency of the reference clock generator 8, in other words, it will also follow the reference clock frequency from a base station which is superposed on a receiving signal.
Next, as the second related art, the WCDMA system cellular phone by a direct conversion system will be described with reference to FIG. 11.
As shown in FIG. 11, the WCDMA system cellular phone includes an antenna 201, a duplexer (DUP) 202 for performing simultaneous sending and receiving using the single antenna 201, a reference clock generator (TCXO) 203 and an accumulator 204. The WCDMA system cellular phone also includes a frequency offset estimator 205, a delay profile searcher 206, a timing generator 207, a low noise amplifier (LNA) 211 and band-pass filters (BPF) 212, 222, 215 and 225. The WCDMA system cellular phone further includes a quadrature demodulator (DEM) 213, AGC amplifiers 214, 224, an A/D converter 216, PLLs 217, 227, a finger circuit 218, a RAKE circuit 219, a power amplifier (PA) 221, a quadrature modulator (MOD) 223 and a D/A converter 226. The WCDMA system cellular phone further more includes a channel codec 228, and a speech processor, a codec circuit, a speaker and a microphone which are not illustrated.
Here, a receiving circuit is composed of the low noise amplifier 211, the band-pass filter 212, the quadrature demodulator 213, the AGC amplifier 214, the band-pass filter 215, the A/D converter 216, the finger circuit 218, the RAKE circuit 219 and the frequency offset estimator 205. The receiving circuit is also composed of the delay profile searcher 206, the timing generator 207, the accumulator 204, the reference clock generator 203 and the PLL 217.
A transmitting circuit is composed of the channel codec 228, the D/A converter 226, the band-pass filter 225, the quadrature modulator 223, the AGC amplifier 224, the band-pass filter 222, the power amplifier 221, the accumulator 204, the reference clock generator 203 and the PLL 227.
First, each part which composes the receiving circuit will be described. As shown in FIG. 11, the PLL 217 generates a local frequency signal for the quadrature demodulator 213 using output of the reference clock generator 203 as a reference clock. A receiving signal, received at the antenna 201 as a downlink signal (a reception high-frequency signal) from a base station, is led to the receiving circuit by the duplexer 202. In the receiving circuit, the receiving signal is amplified by the low noise amplifier 211. And, among amplified signals, only a high-frequency signal in a required bandwidth is selected by the band-pass filter 212. After that, in the quadrature demodulator 213, the quasi-synchronous demodulation is performed with a local frequency signal supplied from the PLL 217, and the selected high-frequency signal is converted into a reception analog baseband signal. And, the converted reception analog baseband signal is converted into a reception digital baseband (a reception DBB) signal in the A/D converter 216. The converted reception digital baseband signal is input to the delay profile searcher 206 and the finger circuit 218. Here, a reference clock from a base station is superposed on the reception digital baseband signal.
Also, the delay profile searcher 206 compares a frame timing signal given from the timing generator 207 and the input reception digital baseband signal, generates a frame timing correction time and outputs that to the timing generator 207. The timing generator 207 generates an ideal frame timing signal by using the reference clock output by the reference clock generator 203. And, the timing generator 207 generates the frame timing signal, by performing correction adding the frame timing correction time input from the delay profile searcher 206 to the generated ideal frame timing signal, and supplies the frame timing signal to the delay profile searcher 206 and the finger circuit 218. When the frame timing signal is input newly, the delay profile searcher 206 calculates a difference in timing and supplies that to the timing generator 207 as a new frame timing correction time. By repeating such correction process, an accurate frame timing signal is always supplied to the delay profile searcher 206 and the finger circuit 218.
The finger circuit 218 includes a plurality of fingers for separating receiving signals being dispersed and delayed in multipath components. The finger circuit 218 demodulates the reception digital baseband signal in each finger, based on the corrected frame timing signal, and sends outputs of the fingers to the RAKE circuit 219. The finger circuit 218 also extracts pilot data included in the reception digital baseband signal with respect to each finger, and outputs to the frequency offset estimator 205. The frequency offset estimator 205 calculates a frequency offset value for each finger, based on the pilot data with respect to each finger input from the finger circuit 218, and outputs the frequency offset value for each finger to the RAKE circuit 219. By performing weighted synthesis of the output signal for each finger from the finger circuit 218, which is based on the frequency offset value for each finger input from the frequency offset estimator 205, the RAKE circuit 219 generates a reception demodulated data. As a result, the reception demodulated data in which the fading effect has been reduced is generated.
The frequency offset estimator 205 also performs weighted synthesis of the frequency offset value for each finger of the finger circuit 218 and outputs that to the accumulator 204 as a synthetic frequency offset value. The accumulator 204 adds the input synthetic frequency offset value and the present output value, and outputs the addition result to a frequency control terminal of the reference clock generator 203. The reference clock generator 203 changes an oscillation frequency according to the output value of the accumulator 204 which is input to the frequency control terminal. In this way, the output frequency of the reference clock generator 203 follows the reference clock frequency from a base station which is superposed on a receiving signal and changes (frequency capture and frequency following).
Next, each part which composes the transmitting circuit will be described. The PLL 227 generates a local frequency signal for the quadrature modulator 223 by using output of the reference clock generator 203 as a reference clock. Transmitting data is converted by the D/A converter 226 into a transmission analog baseband signal after encoded by the channel codec 228. Further, the transmission analog baseband signal is converted into a transmission signal, as an uplink signal (a transmission high-frequency signal), with a local frequency signal supplied from the PLL 227 in the quadrature modulator 223. The transmission signal is led to the antenna 201 by the duplexer 202 after amplified by the AGC amplifier 224 and the power amplifier 221. And, the transmission signal is transmitted from the antenna 201 as an uplink signal (a transmission high-frequency signal) from the mobile station to a base station.
According to aforementioned constitution, because a local frequency signal supplied to the quadrature modulator 223 is generated by the PLL 227 which uses output of the reference clock generator 203 as a reference clock, a frequency of the uplink signal (the transmission high-frequency signal) also changes according to the output frequency of the reference clock generator 203. Therefore, a frequency of the uplink signal (the transmission high-frequency signal) can follow the reference clock frequency from a base station which is superposed on the reception high-frequency signal (a downlink signal).
In a mobile radio communication system, the quasi-synchronous demodulation is performed for a receiving signal at both of a base station and a mobile station. And, because the mobile station makes its own reference clock frequency follow the reference clock frequency from the base station on communicating as mentioned above, any difference in frequency in bidirectional data communication of reception and transmission between the base station and the mobile station can be suppressed. As a result, speeding up of signal demodulation processing after quasi-synchronous demodulation of the receiving signal can be achieved, and improvement of the signal transmission throughput can also be achieved.
The reference clock generator used in the base station is more stable under temperature change and vibration than the one used in the mobile station. This is the reason why the reference clock frequency of the mobile station is configured to follow the receiving signal (a downlink signal) frequency from the base station. It is to improve frequency stability of the entire system that the mobile station makes its own reference clock frequency follow the receiving (downlink) signal frequency which is generated based on the reference clock in the base station.
Next, Doppler effect which occurs when a mobile station moves at high speed like a mobile radio communication system, and its influence will be described.
First, the basis of Doppler effect will be described. When an electromagnetic source is moving at speed v [m/s] in the direction of angle θ seen from an observer, frequency f [Hz] of an electromagnetic wave observed by the observer is expressed in the following formula.
                    f        =                                                            1                -                                                      (                                          v                      /                      c                                        )                                    2                                                                    1              -                                                (                                      v                    /                    c                                    )                                ⁢                cos                ⁢                                                                  ⁢                θ                                              ⁢                      f            0                                              (        1        )            Here, fo is a frequency of the electromagnetic wave oscillated by the electromagnetic source, v is the moving speed of the electromagnetic source seen from the observer, c is light speed, and θ is an angle of the moving direction of the electromagnetic source seen from the observer.
It makes θ=0 where the case that the electromagnetic source is coming toward the observer. The frequency f at this time is calculated by substituting θ=0 in the formula (1), and the result becomes as follows.
                    f        =                                                                              1                  -                                                            (                                              v                        /                        c                                            )                                        2                                                                              1                -                                  (                                      v                    /                    c                                    )                                                      ⁢                                                  ⁢                          f              0                                =                                                                      1                  +                                      (                                          v                      /                      c                                        )                                                                                                1                  -                                      (                                          v                      /                      c                                        )                                                                        ⁢                          f              0                                                          (        2        )            
Here, if it is given v<<c, it can be approximated as below.√{square root over (1−(v/c)2)}≈1  (3)
That is, the formula (2) can be approximated by the formula (4) as below.
                    f        =                                                                              1                  +                                      (                                          v                      /                      c                                        )                                                                                                1                  -                                      (                                          v                      /                      c                                        )                                                                        ⁢                                                  ⁢                          f              0                                =                                                                      1                  +                                      (                                          v                      /                      c                                        )                                                                                        1                    -                                                                  (                                                  v                          /                          c                                                )                                            2                                                                                  ⁢                                                          ⁢                              f                0                                      ≈                                          [                                  1                  +                                      (                                          v                      /                      c                                        )                                                  ]                            ⁢                              f                0                                                                        (        4        )            
Further, in the following description, the influence of Doppler effect is described using the formula (1) or it is described using the approximate formula (4) for easiness.
Next, the influence of Doppler effect to a mobile radio communication system will be described.
For example, as shown in FIG. 12A, it is supposed that three base stations 901, 902 and 903 are arranged from the left side in the figure by this order on one-dimensional space. These base stations 901, 902 and 903 are base stations which transmit downlink signals in all identical frequency fo [Hz].
As shown in FIG. 12A, a mobile station 904 is locating between the base station 901 and the base station 902, and it is supposed that it moves away from the base station 901 at high speed of speed v [m/s] and it comes closer to the base stations 902 and 903 at same high speed. In this case, in the mobile station 904, it is observed as if the downlink signal frequency fo [Hz] from each base stations 901, 902 and 903 were changing as shown in FIG. 12B due to Doppler effect. That is, when seeing from the mobile station 904 which is moving away, the downlink signal frequency fo from the base station 901 is observed as if shifting to decreased direction. At that time, when the approximate formula (4) is used, the downlink signal frequency fb [Hz] from the base station 901 observed at the mobile station 904 is represented by the following formula (5) and FIG. 12B. On the other hand, when seeing from the mobile station 904 which is approaching the base station 902 at high speed, the downlink signal frequency fo from the base station 902 is observed as if shifting to increased direction. At that time, when the approximate formula (4) is used similarly, the downlink signal frequency fd [Hz] from the base station 902 observed at the mobile station 904 is represented like the following formula (6) and FIG. 12B. Similarly, when seeing from the mobile station 904 which is approaching, the downlink signal frequency fo from the base station 903 is observed as if shifting to the increased direction. At that time, the downlink signal frequency fa [Hz] from the base station 903 observed at the mobile station 904 is also represented like the formula (6) and FIG. 12B.fb=(1−v/c)·fo  (5)fd=fa=(1+v/c)·fo  (6)
where, c is light speed.
In other words, a frequency of a downlink signal from a base station observed at a mobile station increases or decreases by v/c of frequency ratio compared with a frequency of a downlink signal received during a time of static condition. As a result, the reference clock frequency inside the mobile station also changes by v/c of the same frequency ratio in the configuration of the mobile radio communication system in which the reference clock to frequency of the mobile station follows the reference clock frequency of the base station as mentioned above.
Further, transmitting and receiving an uplink signal and a downlink signal are generally performed by using the same base station in a mobile radio communication system. For this reason, under the system which uses a receiving signal quasi-synchronous demodulation system, when frequency shift by v/c of frequency ratio due to Doppler effect occurs to a downlink signal, the frequency shift by further equivalent amount of frequency in an uplink signal is observed at a base station. It will be described specifically with reference to FIG. 12A about this. As shown in FIG. 12A, it is supposed that the mobile station 904 which is approaching the base station 902 at high-speed of v [m/s] is receiving a downlink signal from the base station 902. At this time, the reference clock frequency inside the mobile station 904 is increasing (1+v/c) times of the reference clock frequency at a time of static condition due to Doppler effect.
The mobile station 904 generates an uplink signal in this state. Transmitting frequency from the mobile station side changes according to the output frequency of the reference clock generator. Therefore, the uplink signal whose transmitting frequency is increased by (1+v/c) times of the transmitting frequency at a time of static condition is generated. When the base station 902 receives the uplink signal whose frequency is being increased by (1+v/c) times of a frequency at a time of static condition, a frequency which is further increased by (1+v/c) times is observed at the base station 902 because Doppler effect also influences to an uplink direction. In other words, when the mobile station 904 is approaching the base station 902 at high speed (v [m/s]), a frequency having (1+v/c) squared times of the frequency fo, which is observed at the base station 902 when the mobile station 904 is in a static condition, is observed at the base station 902.
The influence by Doppler effect by a mobile station which is moving at high speed also causes the following phenomenon. That is, it is supposed that the mobile station 904 passed the base station 902, and a positional relationship between the mobile station 904 and the base station 902 has changed from a position directional relation shown in FIG. 12A to a position directional relation shown in FIG. 13A. At that time, frequency shift by Doppler effect suddenly changes to a decreased direction shift in FIG. 13A while it was a increased direction shift in FIG. 12A. The observed frequency fd′ at the mobile station 904 when the mobile station 904 has passed the base station 902 is given by the formula (7). At that time, frequency variation Δ fd observed at the mobile station 904 is given by the formula (8).fd′=fb=(1−v/c)·fo  (7)Δfd=−(2·v·fo)/c  (8)
Similarly, when a position directional relation between the mobile station 904 and the base station 902 has changed to a relation from FIG. 12A to FIG. 13A, a frequency observed at the base station 902 suddenly changes from (1+v/c)·fo to (1−v/c)·fo. Accordingly, the frequency variation value observed at the base station 902 becomes −(4·v·fo)/c because the frequency variation value of −(2·v·fo)/c observed at the mobile station 904 is added to the variation value from (1+v/c)·fo to (1−v/c)·fo.